The present invention is about a sol-gel process for the manufacture of nanocomposite materials being photoluminescent at ambient temperature and the materials obtained through this process. The term xe2x80x9cnanocompositexe2x80x9d has recently become of common use in the field of materials science to define bodies or films having the feature of containing at least one material in grains or crystals having dimensions in the range of the nanometers; generally the grains of the material in nanometric dimension are dispersed in a matrix of a different material.
The phenomenon of photoluminescence at ambient temperature in silicon was discovered in 1990 in porous silicon obtained by anodic etching of a silicon wafer. Although the physical principles underlying the phenomenon are not fully understood yet, it seems that this may be ascribed, at least partially, to nanostructural features of the material and the resulting quantum effects associated to the confinement to extremely reduced distances, in the range of nanometers (nm), of the motion of charge carriers such as electrons and holes. Nanocomposite materials of suitable composition have also been known for some time for their non-linear optical properties and for their luminescence. Nanocomposite photoluminescent materials have a potential application as light sources, and the ones based on silicon, if their production proves to be compatible with silicon technology, may be used in electronic devices as well. Possible uses in the optoelectronic field are also foreseen for these materials, thanks to the possibility of modulating the dielectric properties of silica or of enhancing the emission properties of radiative centres, such as Er3+ ions in silica. It is also studied the possible use of these materials for the production of extremely high density memories, through the realization of single-electron transistors, wherein the electron is confined in a space having dimensions of few nanometers buried in an insulating matrix, for instance silica, SiO2.
As a consequence of these possible applications, a strong interest exists in the industry in the possibility of obtaining nanocomposite materials of controlled characteristics, in a way that is both reproducible and compatible with the technologies typical of the semiconductor industry, and in particular with the technology of silicon.
Several techniques have been proposed for the production of nanocomposite materials, generally in the form of particles of silicon or silicon carbide with dimensions in the range of nanometers in a matrix of silica.
A first synthesis route is described in an article of Chawet et al., Journal of Applied Physics, vol. 85, April 1999, No. 8, pages 4032-4039. According to the method of this article, a composite material comprising silicon particles having dimensions of nanometers in SiO2 is produced by co-sputtering of a SiO2 target and several pieces of Si each having a surface of 1 cm2. Sputtering and modifications thereof (among which co-sputtering) are techniques very well known in the field of materials science and microelectronic industry. By these methods it is possible to produce nanocomposite materials in the shape of thin layers, having a thickness generally lower than about 1 micron, over an inert support. According to the contents of the cited article, nanocomposite layers thus produced become photoluminescent only after a thermal treatment at temperatures in excess of 900xc2x0 C.
An article of Mutti et al., Applied Physics Letters, vol. 66, February 1995, No. 7, pages 851-853, describes the production by ionic implantation of Si+ ions in SiO2 of nanocomposite materials made up of silicon particles with dimensions in the range of nanometers in silica. In this case too the product of ion implantation must be thermally treated at temperatures of at least 1000xc2x0 C. in order to observe the appearance of photoluminescence.
The methods of the two articles above are suitable for research purposes but, due to low productivity, cannot be exploited in industrial productions.
A chemical synthesis route of nanocomposite materials has been proposed recently. An article of Guangining Li et al., Applied Physics Letters, vol. 76, June 2000, No. 23, pages 3373-3375, describes a sol-gel synthesis of nanocomposite photoluminescent materials consisting of SiC particles in silica. According to the teachings of this article, a sol-gel synthesis is carried out using a standard silica precursor, e.g. tetraethylorthosilane (TEOS), to which a modifier component is added. Said modifier component is chosen among silicon alkoxides wherein the xe2x80x94OR radicals are partially substituted by aromatic hydrocarbon radicals; an example of such modified alkoxides is diethoxydiphenylsilane. The modifier component may also be chosen among aromatic compounds with condensed rings (such as anthracene) or conjugated rings (such as stilbene). The modifier component may be added to TEOS in the starting solution; alternatively, the synthesis may be started with TEOS alone and, once a wet silica gel is obtained, its pores may be soaked with the modifier component. The thus obtained gels are then dried and treated at temperatures comprised between about 800 and 1000xc2x0 C. in an atmosphere of air or nitrogen. This synthesis route is an improvement from the industrial standpoint compared to previous methods, but still has some drawbacks: alkoxides substituted with aromatic radicals are reagents of rather high cost, while conjugated- or condensed-rings aromatic compounds are only sparingly soluble in the hydroalcoholic solutions used in the first steps of sol-gel processes thus posing homogeneity problems; besides, aromatic hydrocarbons or compounds containing aromatic hydrocarbon radicals are generally carcinogenic, their industrial use thus being potentially dangerous and undesirable.
It is thus an object of the present invention to provide a sol-gel process free of the drawbacks of the prior art for the manufacture of nanocomposite materials, based on the silicon/silica combination, being photoluminescent at ambient temperature; a further object of the invention is to provide the materials obtained through this process.
These objects are achieved according to the present invention with a sol-gel process comprising the following steps:
preparing an aqueous or hydroalcoholic mixture containing a silicon alkoxide, an additional component A, and an acidic catalyst, wherein the molar ratio between water molecules and silicon atoms is equal to or higher than 4;
causing the mixture to gel obtaining a wet gel;
causing said wet gel to dry; and
densify the thus obtained dry gel by means of a thermal treatment having a maximum temperature comprised between 1200xc2x0 C. and 1400xc2x0 C.;
characterized in that:
the additional component A is a dialkyldialkoxysilane, R2xe2x80x94Sixe2x80x94(ORxe2x80x2)2, or an alkyltrialkoxysilane, Rxe2x80x94Sixe2x80x94(ORxe2x80x2)3, wherein R and Rxe2x80x2 radicals are not aromatic; and
in the range from 300 to 800xc2x0 C. the thermal treatment is carried out under an atmosphere made up of pure HCl or a mixture containing at least 5% by volume of HCl in an inert gas, said atmosphere being anhydrous and not containing oxygen.
The inventors have found that by using as the additional component A an alkylalkoxysilane as one of the starting reagents in the sol-gel process, and treating in the range from 300 to 800xc2x0 C. the dry gel obtained as an intermediate product of the process with an atmosphere of HCl (or HCl mixed with an inert gas) not containing water and oxygen, a black, non-transparent material is obtained that, analyzed with spectrophotometric methods, shows the presence of elemental silicon as well as intense emission bands in the IR range, with a tail in the red part of the visible region of the electromagnetic spectrum.